The turbine of an axial flow gas turbine engine conventionally comprises at least one annular array of radially extending rotor aerofoil blades located in the primary motive fluid passage of the engine. The radially outer extents of the blades are surrounded in radially spaced apart relationship by an annular sealing member attached to the casing of the turbine. The radial distance between the blades and the sealing member is desirably as small as possible in order to minimise the leakage of motive fluid gases past the rotor blades: the greater the leakage of gases, the lower the efficiency of the turbine.
Unfortunately during a typical gas turbine engine operating cycle, rotational speed and temperature variations within the turbine result in significant variation of the radial clearance between the blades and the sealing member. Accordingly in order to ensure that damaging contact does not occur between the blades and sealing member, the clearance between them has to be larger than would otherwise be desirable for certain engine operating conditions.
The condition which results in the smallest clearance between the blades and sealing member occurs when the gas turbine engine is suddenly brought up to full power. Typically this occurs during the take-off of an aircraft powered by the engine. Under these conditions the blades heat up rapidly and so thermally expand. Additionally their rotational speed increases so that they are subjected to centrifugal growth. At the same time the sealing member and the casing which supports it heat up rapidly and so thermally expand.
The rate of thermal expansion of the casing and the blades and associated structure are desirably matched so that the rotor blade/sealing member radial gap remains within acceptable limits. This is achieved by the so-called "slugging" of the turbine casing. "Slugging" is the positioning of slugging masses or thermal barriers on the casing to modify its thermal expansion behaviour.
When the gas turbine engine assumes a steady state, typically under cruise conditions, a temperature equilibrium situation is reached. However, the equilibrium temperature reached by the various components of the turbine are such that the radial gap between the turbine blades and their associated sealing member is larger than would otherwise be desirable.
Attempts have been made to overcome the problem of variation in the radial gap between the sealing member and the blades by the provision of intermittent cooling of the turbine casing. Typically the casing is uncooled during take-off to ensure that the radial gap remains within acceptable limits. However when cruise conditions are reached, casing cooling is commenced to reduce the radial clearance between the sealing member and the turbine blades to an optimum value.
One drawback with this arrangement is that since the turbine casing is modified by slugging to slow down its thermal response rate, it is equally slow to respond to the effects of deliberate cooling.
A further drawback is that the turbine casing must be made from an alloy which is sufficiently resistant to the high temperatures which it is likely to reach when it is not cooled.